a) Field of the Invention
b) Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively are needed to drive the displays. In contrast, an array of thin film transistors (TFT) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate are required for the active type EPDs. An electrophoretic fluid composed of a colored dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrodes.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos. 5,961,804, 5,930,026 and 6,017,584). Each of these has its own problems as noted below.
In a partition-type EPD, there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of the particles such as sedimentation. However, difficulties are encountered in the formation of the partitions, the process of filling the display with a fluid, enclosing the fluid in the display, and keeping the suspensions of different colors separated from each other.
The microencapsulated EPD has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a dispersion of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules can be formed by interfacial polymerization, in-situ polymerization or other known methods such as physical processes, in-liquid curing or simple/complex coacervation. The microcapsules are typically prepared in an aqueous solution and, to achieve a useful contrast ratio, their mean particle size is relatively large (50-150 microns). The large microcapsule size results in a poor scratch resistance and a slow response time for a given voltage because a large gap between the two opposite electrodes is required for large capsules. Also, the hydrophilic shell of microcapsules prepared in an aqueous solution typically results in sensitivity to high moisture and temperature conditions. If the microcapsules are embedded in a large quantity of a polymer matrix to obviate these shortcomings, the use of the matrix results in an even slower response time and/or a lower contrast ratio. To improve the switching rate, a charge-controlling agent is often needed in this type of EPDs. However, the microencapsulation process in an aqueous solution imposes a limitation on the type of charge-controlling agents that can be used. Other drawbacks associated with the microcapsule system include poor resolution and poor addressability for color applications.
The first aspect of the present invention is directed to an improved electrophoretic display comprising isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. The cells are individually sealed.
Another aspect of the invention relates to a novel process for the manufacture of such an electrophoretic display.
A further aspect of the invention relates to the preparation of cells of well-defined shape, size and aspect ratio. The cells enclose a suspension of charged pigment particles dispersed in a dielectric solvent and are formed from microcups prepared according to the present invention. In one of the preferred embodiments of this invention, the process for the preparation of the microcups involves embossing a thermoplastic or thermoset precursor layer coated on a conductor film with a pre-patterned male mold, followed by releasing the mold during or after the thermoset precursor layer is hardened by radiation, cooling, solvent evaporation, or other means. Alternatively, in another preferred embodiments of this invention, the microcups may be formed from imagewise exposure of the conductor film coated with a radiation curable layer followed by removing the unexposed areas after the exposed areas have become hardened. In another preferred embodiments of this invention, the microcups may be formed by laminating a spacer film having pre-punched holes onto the conductor film. The adhesion between the pre-punched spacer film and the conductor film may be improved by using an adhesive and a good solvent for the spacer film. Heat or UV radiation may also be applied to enhance the adhesion.
Solvent-resistant, thermomechanically stable microcups having a wide range of size, shape, and opening ratio can be prepared by any of the aforesaid methods. The microcups are then filled with a suspension of charged pigment particles in a dielectric solvent, and sealed.
Yet another aspect of the present invention relates to the sealing of the microcups filled with the electrophoretic fluid containing a dispersion of charged pigment particles in a dielectric fluid. Sealing can be accomplished by a variety of ways. In one of the preferred approaches, it is accomplished by dispersing a thermoplastic or thermoset precursor in the electrophoretic fluid before the filling step. The thermoplastic or thermoset precursor is immiscible with the dielectric solvent and has a specific gravity lower than that of the solvent and the pigment particles. After filling, the thermoplastic or thermoset precursor phase separates from the electrophoretic fluid and forms a supernatant layer at the top of the fluid. The sealing of the microcups is then conveniently accomplished by hardening the precursor layer by solvent evaporation, interfacial reaction, moisture, heat, or radiation. UV radiation is the preferred method to seal the microcups, although a combination of two or more curing mechanisms as described above may be used to improve the yield and throughput of the sealing step.
Alternatively, the sealing can be accomplished by overcoating the electrophoretic fluid with a solution containing the thermoplastic or thermoset precursor, particularly when the sealing precursor is at least partially compatible with the dielectric solvent. The sealing is then accomplished by hardening the precursor by solvent evaporation, interfacial reaction, moisture, heat, radiation, or a combination of curing mechanisms. Interfacial polymerization followed by UV curing is very beneficial to this sealing process. Intermixing between the electrophoretic layer and the overcoat is significantly suppressed by the formation of a thin barrier layer at the interface by interfacial polymerization. The sealing is then completed by a post curing step, preferably by UV radiation. To further reduce the degree of intermixing, it is highly desirable that the specific gravity of the overcoating is significantly lower than that of the electrophoretic fluid. The two-step overcoating process is particularly useful when the dye used is at least partially soluble in the sealing material. Additives such as surfactants and filler particles may also be used to further enhance the properties of the sealing layer. The sealing processes are especially unique features of the present invention.
Yet another aspect of the present invention relates to a multiple step process for the manufacture of a monochrome electrophoretic display comprising isolated cells having well-defined shape, size and aspect ratio. The processing steps include preparation of the microcups by any one of the methods described above, filling the microcups with an electrophoretic fluid, sealing and isolating the filled microcups, and finally laminating the sealed and isolated microcups with a second conductor film pre-coated with an adhesive layer. This multiple-step process can be carried out roll-to-roll continuously.
Yet another aspect of the present invention relates to a process for the manufacture of a full color electrophoretic display by laminating or coating the preformed microcups with a layer of positively working photoresist, selectively opening a certain number of the microcups by imagewise exposing the positive photoresist, followed by developing the resist, filling the opened cups with a colored electrophoretic fluid, and sealing the filled microcups by a sealing process. These steps may be repeated to create sealed microcups filled with electrophoretic fluids of different colors.
The microcup structure and the sealing processes enable a format flexible, efficient roll-to-roll continuous manufacturing process for the preparation of EPDs. The displays can be prepared on a continuous web of a conductor film such as ITO/PET by, for example, (1) making the microcup structure on a conductor film by the microembossing or photolithographic method, or by laminating with a spacer film having pre-punched holes, (2) filling the microcups with an electrophoretic fluid and sealing the microcups, (3) laminating the sealed microcups with the other conductor film, and (4) slicing and cutting the display to a desirable size or format for assembling.
These multiple-step processes as disclosed may be carried out roll-to-roll continuously or semi-continuously. Consequently, they are suitable for high volume and low cost production. These processes are also efficient and inexpensive as compared to other processes for high volume production operations.
One of the major advantages of the EPDs of the present invention, is that the microcup wall is in fact a built-in spacer to keep the top and bottom substrates apart at a fixed distance. The mechanical properties and structural integrity of microcup displays are significantly better than any prior art displays including those manufactured by using spacer particles. In addition, displays involving microcups have desirable mechanical properties including reliable display performance when the display is bent, rolled, or under compression pressure from, for example, a touch screen application. The use of the microcup technology also eliminates the need of an edge seal adhesive which would limit and predefine the size of the display panel and confine the display fluid inside a predefined area. The display fluid within a conventional display prepared by the edge sealing adhesive method will leak out completely if the display is cut in any way, or if a hole is drilled through the display. The damaged display will be no longer functional. In contrast, the display fluid within the display prepared by the microcup technology is enclosed and isolated in each cell. The microcup display may be cut to almost any dimensions without the risk of damaging the display performance due to the loss of display fluid in the active areas. In other words, the microcup structure enables a format flexible display manufacturing process, wherein the process produces a continuous output of displays in a large sheet format which can be cut to any desired format. The isolated microcup or cell structure is particularly important when the cells are filled with fluids of different specific properties such as colors and switching rates. Without the microcup structure, it will be very difficult to prevent the fluids in adjacent areas from intermixing, or being subject to cross-talk during operation.
The electrophoretic display prepared according to the present invention is not sensitive to environment, particularly humidity and temperature. The display is thin, flexible, durable, easy-to-handle, and format-flexible. Since the electrophoretic display prepared according to the present invention comprises isolated cells of favorable aspect ratio and well-defined shape and size, the bi-stable reflective display has excellent color addressability, high contrast ratio, and fast switching rate. The drawbacks of electrophoretic displays prepared by the prior art processes are therefore all eliminated.